Riders

Three of the Four Horsemen of the Apocalypse - Death, Famine, and Pestilence (all &) - appear in NetHack as monsters on the Astral Plane. They are collectively known as The Riders, even though they are not riding anything. However, the name "Riders" has existed since before riding was implemented.

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Riders can fly/float, are humanoid, can regenerate themselves, can see invisible creatures, and have teleport control. They can also unlock doors.

All riders come back to life after being killed: after 12 turns, a rider corpse has a 1/3 chance of revival on each turn[1]. Eating their corpses doesn't get rid of them either; it gets rid of you quite effectively, though. (This is a YASD that usually occurs only once.) You cannot polymorph their corpse, either. Their corpses will instantly revive if you try to pick them up, tin them, push a boulder on their square, teleport them, etc.

Teleportation has a special effect on riders: if they are zapped with a wand of teleportation, they are teleported to a square adjacent to you with probability 12/13[2]. This is a very Bad Idea. Attempting to teleport or tin their corpses is also a bad idea as they will be immediately revived.

The Riders are not disintegration-resistant, per se, in the way that black dragons are; however, attempting to disintegrate one of them will cause it to re-integrate instantly.

Although the normal way to get the Riders off your back is simply to ascend, there are a few trickier ways.

The primary way to permanently banish them is to kill them, then fill every square on the level with monsters so that when they revive, there is nowhere for them to go. When you see the message "You feel less hassled"[3], a Rider corpse has started to decay, and will eventually rot away, provided that you do nothing to disturb it.

A hostile gelatinous cube can "eat" their corpse but not digest it, placing it into the cube's inventory. You could then steal it as a nymph. To destroy it, you can repeatedly loot a cursed bag of holding in 0 turns, or a number of other methods. If you try to sacrifice it, there is a good chance it will revive on your way to the coaligned altar. The Rider corpses will give you severe encumberance problems. If you are completely unable to lift 1450 units, the corpse will drop uselessly to the floor.

Riders have 100% magic resistance, which makes them immune to polymorph, but if you polymorph yourself into a green slime, you can turn them to slime because your sliming attack is not considered magical. If you want, you can then tame the resulting green slimes. Similarly, a substantial herd of tamed green slimes stands some chance of turning a Rider into a slime, although this only somewhat less risky.

Famine and Pestilence can be tamed with charm monster if they are level-drained first (note that they will resist the spell drain life). Death is immune to level drain, but in SLASH'EM, you can polymorph yourself into a genetic engineer, then polymorph him with your special attack (like sliming, this is not negated by magic resistance), then tame whatever he turns into. He will eventually revert back to his normal form, which will remain tame.

In the game, only Death, Famine, and Pestilence are named. If you #chat to one of them, you are told, "Who do you think you are, War?", though the wording makes the meaning ambiguous. However, a comment in the source[4] is definitely not ambiguous:

/* Riders -- the Four Horsemen of the Apocalypse ("War" == player)
*/

Also, attempting to tin the corpse of a Rider gives the message, "Yes.. but War does not preserve its enemies."

Famine and Pestilence are both vulnerable to death rays (Death absorbs them and increases his maximum HP), and all three Riders are vulnerable to magic missiles.

Of the three riders, Pestilence is widely considered to be the most dangerous. Hence a common Astral Plane strategy is to identify which altar is guarded by Pestilence using telepathy, and then explore that altar last.

Thrown potions of (extra/full) healing damage Pestilence and reduce his maximum hit points. Thrown or quaffed potions of sickness heal him. Pestilence may spawn carrying potions of sickness; the first time you kill him, you should pick these up and dispose of them.

The horsemen are characters in Revelation, the last book of the Bible.

War:I heard the second living creature say, "Come!" Then another horse came out, a fiery red one. Its rider was given power to take peace from the earth and to make men slay each other. - Revelation 6:4

Famine:I looked, and there before me was a black horse! Its rider was holding a pair of scales in his hand. Then I heard what sounded like a voice among the four living creatures, saying, "A quart of wheat for a day's wages, and three quarts of barley for a day's wages, and do not damage the oil and the wine!" - Revelation 6:6

Death:I looked, and there before me was a pale horse! Its rider was named Death, and Hades was following close behind him. - Revelation 6:8

Note that the remaining horseman (described as the first in Revelation) is ordinarily interpreted to be the Antichrist. The idea of Pestilence as a horseman is probably derived from passages following the description of Death, but is not actually a part of the Biblical prophecies.

They were to have met in the garden of the Chapelle Expiatoire at five o'clock in the afternoon, but Julio Desnoyers with the impatience of a lover who hopes to advance the moment of meeting by presenting himself before the appointed time, arrived a half hour earlier.

So opens The Four Horsemen of the Apocalypse by Vicente Blasco Ibanez, as translated from Spanish to English by Charlotte Brewster Jordan. The book is in the public domain; you can read it at Google Books. Since the mention of the four horsemen in the Bible, they have appeared in many other places: in the book quoted above, in the books' various motion film versions, and also in roguelike games including ToME and NetHack.

GreyKnight has written a rudimentary patch which occasionally replaces the Riders with some or all of the Seven Deadly Sins. The original hosting site has been taken over by a domain squatter, but the patch is once again available, hosted directly on Bilious.

[Pestilence:] And I saw when the Lamb opened one of the seals,
and I heard, as it were the noise of thunder, one of the four
beasts saying, Come and see. And I saw, and behold a white
horse: and he that sat on him had a bow; and a crown was given
unto him: and he went forth conquering, and to conquer.

[War:] And when he had opened the second seal, I heard the
second beast say, Come and see. And there went out another
horse that was red: and power was given to him that sat thereon
to take peace from the earth, and that they should kill one
another: and there was given unto him a great sword.

[Famine:] And when he had opened the third seal, I heard the
third beast say, Come and see. And I beheld, and lo a black
horse; and he that sat on him had a pair of balances in his
hand. And I heard a voice in the midst of the four beasts say,
A measure of wheat for a penny, and three measures of barley
for a penny; and see thou hurt not the oil and the wine.

[Death:] And when he had opened the fourth seal, I heard the
voice of the fourth beast say, Come and see. And I looked, and
behold a pale horse: and his name that sat on him was Death,
and Hell followed with him. And power was given unto them over
the fourth part of the earth, to kill with sword, and with
hunger, and with death, and with the beasts of the earth.